1. Field of the Invention
The present invention relates to methods for fabricating oxide-confined vertical-cavity surface-emitting lasers (VCSELs) and, more particularly, to a method for fabricating an oxide-confined VCSEL of enhanced quality by slowing down oxidation to thereby reduce stress caused to an active layer when an oxidation layer is formed.
2. Description of the Prior Art
A conventional process of fabricating an oxide-confined VCSEL typically involves the following steps. Firstly, etching a stacked multi-layer epitaxial substrate in order to form a plurality of trenches on a surface of the substrate. And then, oxidizing one of the stacked layers that is exposed in the trenches so as to define a laser area. However, this process has a drawback, that is, the moisture might invade the layers of the substrate through the trenches, and thus result in failure of actuation of laser.
According to the results of experimental analyses, there are two general causes that may incur the failures for oxide-confined VCSEL devices to emit laser during a composite test involving high temperature, high humidity, and electric current injection. One is the degradation of optical power of the devices, and the other is the cracks formed in the oxidation layer. Further analyses reveal that these failures are basically due to the invasions of moisture in the laser active region and the stress concentrations incurred in the oxidation layer. It is because the invasions of moisture will result in continuous oxidation of the oxidation layer during long-term operation; in addition, during a conventional oxidation process in which the oxidizing rate is generally greater than 0.4 μm/min, stresses are likely to concentrate on the adjacent surfaces of the oxidation layer and thus result in spotty defects. Given lengthy operation, these spotty defects gradually end up as cracks in the oxidation layer. Although, some designs have been disclosed in prior arts to resolve these drawbacks, such like incorporating some features into the laser epitaxial layers in order to reduce the stress concentration of oxidation layer, or optimizing a protective film on the laser window to prevent the moisture from invading. Nevertheless, these prior arts mostly involve in extra or intricate processes and thereby increase the difficulties and costs of the VCSEL fabrication process.